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no little success, and was much resorted to in the middle of the last century. Another practice which is not so rare as one might be inclined to believe is the inducement of measles. Many people, are under the impression that unless children have had all the ordinary exanthematous fevers it is almost desirable that an opportunity should occur for them to have mild attacks of these fevers ; and I have known of instances in which, one out of several children being attacked with measles, no attempt has been made to isolate the sick child, for, it was argued, it was as well for the other children to have the fever also and be done with it as soon as possible. Since this has been done under the influence of a popular belief, I think I am justified in suspecting that the prac- tice of inducing measles for protective purposes is far from un- common, although not generally carried out by professional men. Boeck and Sperino introduced about 1851 the practice of syphili- zation, and these authors recognize clearly that this method is not only a prophylactic, but also a truly therapeutical one. The inducement of a certain disease in order to prevent its recurrence, and even to modify the course of an attack, was therefore a method early recognized in this century both in connection with small- pox and syphilis.

Refractory State produced by Inoculation of an Allied Dis- ease less Fatal, or of the Disease modified by Passage through another Animal. Certain country people had early suspected that a disease affecting cows was communicable to man, and that indi- viduals thus affected were not so liable as other people to small- pox. History tells us that an English farmer and a German schoolmaster in the course of the last century, under the influence of that belief, had resorted to inoculation of that cow disease in preference to the inoculation of true variola. Jenner was the first medical man who discovered the immense importance of these traditional beliefs and practices, and after devoting all his energy to the study of the subject became so convinced of the value of the method of vaccination that after a long struggle he has suc- ceeded in convincing others, and has become thus one of the greatest benefactors of the human race. From 1798 when Jen- ner brought vaccination before the world up to 1880 very little was done to extend the scope of the principle thus discovered. Then Pasteur arose, who, after studying for many years the na- ture of the virus causing several diseases, became gradually con- vinced that this virus may become intensified or attenuated at will, and in 1880 was able to state positively that the production of an attack of definite intensity of many infectious diseases was a thing not only possible, but also practicable and capable of ap- plication for the prevention of disease. In 1880, also, Burdon San- derson suggested that the attenuation of the virus of anthrax for